In recent years, scaling of semiconductor devices has been advanced, and there has been proposed a nonvolatile semiconductor memory device of a three-dimensional structure type that includes memory cells forming a layered structure. According to the nonvolatile semiconductor memory device of the three-dimensional structure type, films to serve as memory elements are stacked in the vertical direction and are processed together, so that a number of memory cells are formed within a unit plane. In this case, the cost per one bit can be reduced.
The nonvolatile semiconductor memory device of the three-dimensional structure type is manufactured as follows. At first, a layered body is prepared by alternately stacking a sacrificial film and a spacer film, each of which is made of an insulating material, to dispose each of them at a plurality of layers, and then memory holes are formed by dry etching in the layered body. Then, a channel and so forth are formed in each memory hole. Thereafter, the sacrificial film is replaced with a conductive material for forming a gate structure in the nonvolatile semiconductor memory device.
When the memory holes are formed, a flux of ions decreases at the bottom side part of each memory hole, and reaction products are not easily removed from the bottom side part of each memory hole. Consequently, the bottom side part of each hole ends up having a tapered shape.